1. Field of the Invention
The present invention relates to a circuit for determining the selection of a filter which separates luminance signal Y and a chrominance signal C from a composite video signal in the digital video signal processing technology, and more particularly to a circuit for detecting an interleave relation between a horizontal synchronous signal and a chrominance subcarrier signal.
2. Description of the Prior Art
FIG. 1 is a block diagram of a conventional signal process for providing the chrominance signal corresponding to the three primary colors of light of red R, green G and blue B by separating the composite video signal in a video system such as a television. That is, a luminance processor 2 and a chrominance processor 3 are connected to an output node of a Y/C separator 1 for separating the luminance and chrominance signals from the composite video signal.
The Y/C separator 1 comprises a low pass filter and a band pass filter so that the luminance signal is extracted through the low pass filter while the chrominance signal is extracted through the band pass filter.
The luminance processor 2 caries out control of contrast and the compensation for contour, and the chrominance processor 3 carries out demodulation of the chrominance signal. The chrominance processor 3 is connected through a burst-processor 4 to the Y/C separator 1 so that the chrominance signal is demodulated on the basis of a burst signal in the demodulation of the chrominance signal.
A synchronous-separator 6 separates horizontal and vertical synchronous signals, which are commonly included in the composite video signal, from each other and provides them to a deflector 7 so that both signals are displayed at a desired position on a picture. A matrix 5 is connected to the luminance processor 2 and to the chrominance processor 3 for carrying out the arithmetic operation, thereby to provide a desired color signal among R, G, and B.
However, in an analog TV receiver as above, the composite video signal is subcarried in a luminance signal band and a modulated chrominance signal is superposed, as shown in FIG. 2(A), so that the perfect separation of the luminance and chrominance signals is very difficult.
FIG. 2(A) shows the Y/C signal bands of the composite video signal. The conventional Y/C separator 1 as shown in FIG. 1 extracts the luminance signal by applying the composite video signal to the low pass filter and also extracts the chrominance signal by applying the composite video signal to the band pass filter. In that case, high frequency components are to be lost in the luminance signal and such high frequency components of the luminance signal are to be included in the chrominance signal band, therefore there is a further problem that a cross color disturbance phenomenon occurs (refer to FIG. 2(B) and (C)).
In order to overcome these problems, a comb filter is adapted which can separates definitely the luminance and chrominance signals from each other. The comb filter can separate the luminance and chrominance signals from each other more definitely if the interleave between the horizontal synchronous signal and the chrominance subcarrier signal is satisfactory, thereby improving the resolution.
Now, with reference to FIG. 2(A), a sub carrier frequency fsc of 3.58 MHz in the composite video signal and a horizontal synchronous frequency fH are related by relation fomular I as follows. ##EQU1## Thus, it represents that the chrominance subcarrier frequency is interleaved according to the horizontal synchronous frequency. And an H-Comb filter is adapted for the separation of luminance and chrominance signals having the above functional relation.
In more detail, FIG. 3(A) and (B) are shown composite video signal lines n and n+1, respectively in which the subcarrier frequency has an odd value, as given by relation formular I, when a present composite video signal is the same with next one. The phase of the chrominance signal including the burst signal is changed approximately 180 degrees, while the luminance signal remains unchanged. Thus, the luminance signal as shown in FIG. 3(C) is obtained by adding the lines together and next dividing the result by 2, while the chrominance signal as shown in FIG. 3(D) is obtained by subtracting one line from the other line and next dividing the result by 2. This Y/C separation method using the H-comb filter prevents the loss of high frequency components of the luminance signal so that it is applied for a high-grade television.
However, the relation fomular I is not accurately applied in home video game systems in the market, or still, in double speed modes of VTR, so that the method of separating the chrominance signal by using the H-comb filter brings about more loss of the image rather than simply using a band pass filter.
Therefore, in order to obtain a better image quality, it is desirable that the chrominance signal be selected from the output of the comb filter if the interleave is satisfactorily kept and that the chrominance signal be selected from the output of the band pass filter if the interleave is not satisfactorily kept in response to discriminating the interleave between the horizontal synchronous signal and the chrominance subcarrier signal.